1. Field of the Invention
This invention relates to electric power conversion, and more particularly relates to methods and devices for controlled power conversion through a resonant charge transfer device.
2. Description of Related Art
Resonant charge transfer devices may be used to formulate a desired output waveform from a given waveform input at the terminals of, for example, a three-phase power device. Generally they operate by transferring a predetermined amount of charge between the various phases of the input terminal and a charge storage device, such as a capacitor, and then, after suitable transfer of charge has occurred, transferring a predetermined amount of charge between the charge storage device and the output terminals. Generally the transfer of charge is mediated through the opening and closing of switches, the operation of which results in a sequential transfer of charge from the various phase inputs of the three-phase input to the charge storage device and then from the charge transfer device to the output. It is known how to predetermine the amount of charge that must be transferred and the precise sequence for doing so to achieve a wide variety of transformations of input power to a desired output power.
FIG. 1 illustrates a functional overview of the operation of a prior art resonant charge transfer device 100. In particular, in converting input AC power 110 to output DC power 170, for example, the circuit generally operates by passing the input power 110 through a stage of input filtering 120, after which charge is transferred to the resonant circuit 130. The amount of charge that is transferred to the resonant circuit 130 is determined by the opening and closing times of input switches 150. In particular, the input switches 150 open and close at predetermined times in accordance with a desired output power to be achieved by the circuit 100. Upon the closing of the input switches 150 after charge has been transferred to the resonant circuit 130, the charge is subsequently permitted to discharge into an output filter stage 140. The manner in which this discharge occurs is determined by the opening and closing of output switches 160. In particular, the output switches 160 open and close at predetermined times in accordance with a desired output to be achieved by the circuit 100. It is known within the prior art how to ascertain the opening and closing times of the input switches 150 and output switches 160 to achieve a desired output power for a given input voltage 110 and output voltage 170.
U.S. Pat. No. 6,118,678, “Charge Transfer Apparatus and Method Therefore,” which is incorporated by reference herein for all purposes, teaches examples of this kind of resonant circuit topology. For example, FIG. 2, taken from U.S. Pat. No. 6,118,678, illustrates a resonant charge transfer device within input terminals 11, an input filter section 10, input switches 20, resonant circuit elements 22,25,26, output switches 30, output filter section 40, and output terminals 12. As taught therein, turning on the switches at predetermined times and operating them so that they self-commutate leads to a wide variety of circuit applications, including but not limited to AC-to-DC rectifier, AC-to-AC power conversion, and DC-to-AC power conversion.
The opening and closing times of the input switches 150 and output switches 160 to achieve a desired output power for a given input power source 110 are a function of the parameters defining the input filter stage 120, the resonant circuit elements 130, and the output filter stage 140. In any particular implementation, however, the parameters of the actual elements comprising the input filter stage 120, resonant circuit elements 130, and output filter stage 140 will deviate from their nominally given values as a function of various factors such as, for example, temperature, operating point, etc. Because the actual parameter values in any particular implementation differ from their nominal values, use of predetermined opening/closing times of the input and output switches 150, 160 will not lead to the precise desired output power; the actual output power 170 will differ from the desired power in some unknown fashion that will vary as the actual parameters differ with temperature, operating point, etc. Hence, a need arises to develop feedback control strategies that actively monitor the operating values of the various circuit parameters as well as the circuit's operating point and control the switching times to achieve the desired output.